(1) Field of the Invention
The present invention relates to a process for the preparation of metal vessels. More particularly, the present invention relates to a process for preparing metal vessels with lap-bonding circumferential end portions of upper and lower members, each consisting of a formed seamless metal cup, to each other through an adhesive, in which generation of a tensile stress in the adhesive layer is prevented, whereby metal vessels excellent in the adhesion and sealing properties of the seam and the durabilities of these properties are produced.
Bottle-like metal vessels obtained by lap-bonding upper and lower members, each consisting of a cup-like article formed by drawing or draw-ironing of a metallic material, on the circumferential open end portions thereof to form a circumferential side seam are advantageous in various points over metal vessels in the form of a can.
(2) Description of the Prior Art
Conventional packaging metal vessels include a so-called three-piece can obtained by double-seaming can lids to the top and bottom of a can barrel having a side seam to form sealed portions and a so-called two-piece can obtained by double-seaming a can lid to the open end portion of a cup-like can barrel formed by drawing or draw-ironing of a metallic material to form a sealed portion.
Metallic vessels having this double-seam structure are much limited in the pressure resistance of the sealed portion or the saving of a metallic material. More specifically, in a seam formed by double seaming, if a load is applied to the seam, the material constituting the seam is first deformed, and by this deformation, leakage from the seam or breakdown of the seam is caused under a relatively small load. In order to avoid this disadvantage, the thickness of the material should be increased. In the field of packaging vessels, from the economical viewpoint and in order to reduce the weight of a vessel, it always is required to reduce the thickness of a metallic material. However, if the thickness of the barrel wall of a vessel is reduced, at the double-seaming step or at the step of preparing for the flanging operation, buckling is readily caused under a load applied in the axial direction of the vessel.
A metallic bottle obtained by lap-bonding the open end portions of upper and lower members, each consisting of a formed cup, is advantageous in that even if the thickness of a seam-constituting material is extremely small, deformation of the material is not caused on the seam by the difference between the outer and inner pressures or the load applied in the axial direction and the seam can resist a load within the critical shear strength thereof irrespectively of the thickness of the material and that since the double-seaming operation need not be performed, the thickness of the side wall of the vessel can be reduced without any risk of occurrence of buckling.
However, in the case where a circumferential side seam is formed by lap-bonding the open end portions of upper and lower members through an adhesive, various problems should be solved so as to provide a strong bonding on the seam and give a reliable seal to the seam.
More specifically, in the lap bonding using an adhesive, no satisfactory bonding force can be obtained unless a certain pressure is applied to an adhesive layer interposed between portions to be bonded. Furthermore, if a stress is left in the adhesive layer, especially if a stress is left in the lapping direction, the adhesion or sealing property is drastically reduced with the lapse of time or under application of an external force.
In the case where a lap seam is straight as in an ordinary can barrel, compression of the lapped portion can be accomplished relatively easily, but when circumferential open end portions of formed cups are lap-bonded, it is very difficult to apply a pressure to the lapped portion.
As means for imparting a pressing force necessary for bonding the lapped portions, there is known a method in which the end portion of a member to be located on the outer side of the seam (often referred to as "outer member" hereinafter) is heated to increase the diameter thereof, a member to be located on the inner side of the seam (often referred to as "inner member" hereinafter) is fitted into the outer member through an adhesive layer, the end portion of the inner member is heated and the end portion of the inner member is thereby expanded in the end portion of the outer member to effect heat bonding through the adhesive (see Japanese Patent Application Laid-Open Specification No. 32228/81).
According to this method, it is possible to press both the end portions of the inner and outer members to the adhesive layer in the molten state, but by this pressing force, the molten adhesive is protruded outward from the lap of both the end portions and the thickness of the adhesive layer is often smaller than the clamping thickness in the normal state. Moreover, since the end portion of the inner member is heated at a higher temperature at the bonding step, the shrinkage of the end portion of the inner member in the radial direction is larger than the shrinkage of the end portion of the outer member. For these reasons, the tensile stress in the radial direction is left in the adhesive layer interposed between both the end portions in the normal state, and deterioration of the adhesion on the interface or leakage from the interface is caused by this tensile stress.